| The magnetic method is an important method to detect the ignition point and the combustion range in the coal field fire zone.The high temperature of the fire zone caused a significant change in the magnetic characteristics of coal and surrounding rocks,which in turn changed the magnetic field in the fire zone.The data-binding theory of these magnetic anomalies was observed at the surface to calculate the distribution of underground coal fires.Therefore,the study of the variation law of the magnetic properties of coal under high temperature and other conditions reveals the mechanism of magnetic anomaly of coal and rock,and provides a theoretical basis for the magnetic method to detect the fire zone.The magnetic susceptibility reflects the variation of the magnetization with the external magnetic field.Therefore,the magnetic characteristics of the coal rock are mainly characterized by measuring the mass susceptibility of the coal rock.In order to further reveal the reasons for these laws,industrial analysis,temperature programming,and X-ray diffraction experiments were conducted to investigate the influence of coal components,gas products,minerals in coal,and coal crystal structure on magnetic properties.(1)The magnetic susceptibility of six kinds of fresh coal samples under different magnetic induction and different particle size conditions was measured by the magnetic balance method using the ancient Egyptian method.The results showed that the coal was weak at the normal temperature and the magnetic induction intensity was higher.High,the weaker the inverse magnetic properties of coal,while the rocks and coal gangue are paramagnetic,and their magnetic susceptibility decreases with the increase of magnetic induction.The susceptibility of coal gangue always lies between coal and rock.The mass susceptibility of coal rock also increases with decreasing particle size.When the particle size is small enough,the coal sample will change from diamagnetic to paramagnetic material.(2)Conduct industrial analysis of the four coal samples,linking the mass magnetic susceptibility of the coal with industrial analysis indicators and found that the ash index and the magnetic susceptibility have the same trend of change.The higher the ash content,the higher the susceptibility of the coal.It shows that the inorganic minerals in coal are the main reasons that affect the magnetic properties of coal.Volatile matter and fixed carbon reflect the content of reverse magnetic substance in coal to a certain extent,and it also has a certain influence on the magnetic properties of coal.(3)In order to study the magnetic changes of burnt coal rocks in the burned area,the original coal rock was first heated and cooled to make a burnt coal rock and measure the susceptibility before and after the temperature rise.The following rules were found:When the burnout temperature reached around 200?,the mass susceptibility of coal became positive,and the magnetic susceptibility began to rise sharply in the range of 300 to 400?,and the growth rate became slower after 500?;Compared with coal gangue,the influence of coal burnt temperature is more obvious,and the susceptibility of the coal is 100~200? earlier than the former.The mass susceptibility of coal that has undergone high temperature burns decreases with the increase of magnetic induction.Small,the rate of decrease is large and small,and the overall susceptibility-magnetic induction intensity curve is quadratic.(4)Simulating the burning coal in the fire zone requires heating and maintaining the coal and rock samples in a high temperature environment.For this reason,the susceptibility measurement system was improved so that it can measure coal samples under high temperature conditions.The experimental results show that the continuous rising temperature of the coal rock is very strong within a short range,and the magnetic susceptibility before and after the magnetic peak will fall to a very low range.The burning of coal rocks can maintain a high magnetic susceptibility for a long time.According to the temperature distribution characteristics of high and low edges in the coal field,the magnetic method can effectively define the range of the fire zone.(5)Through XRD experiments,the mineral composition and coal crystal structure in the coal rock before and after temperature increase were explored.The weak magnetic minerals such as siderite and goethite in coal and rock are transformed into magnetite and other ferromagnetic materials under the influence of high temperature,which is the main reason for the magnetic mutation of coal and rock.High temperatures can affect the stability of the coal crystal structure,but this change has little effect on the magnetic susceptibility and can be ignored. |
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